1. Field
This patent specification describes a switching regulator, and more particularly, a switching regulator capable of performing stable operation over a wide range of operating conditions.
2. Background Art
Recently, a variety of different types of high performance-electrical equipment, such as computer systems and mobile phones, have developed rapidly and come to be used widely. Such electrical equipment requires a power circuit having a high performance, and such a power circuit generally includes a switching regulator to achieve a stable operation.
There are two types of switching regulators: voltage-mode and current-mode switching regulators. A conventional switching regulator generally employs a voltage mode control method. Using the voltage mode control method, the switching regulator uses PWM (Pulse Width Modulation) to stabilize an output voltage by switching the switching device in accordance with a voltage difference between the output voltage and a predetermined reference voltage. However, because a feedback signal is obtained by detecting an output voltage, the switching regulator has several problems, such as slow response speed to input voltage change and a need to use a complex phase compensation circuit.
On the other hand, the current-mode switching regulator has several advantages, such as good liner-regulation and simple phase compensation. Consequently, the current mode switching regulator has come to be widely used recently. However, when an on-duty of the PWM control exceeds 50%, the current-mode switching regulator cannot be controlled due to occurrence of sub-harmonic oscillation, in which the switching regulator oscillates at an integral multiple frequency of a switching frequency. Accordingly, the switching regulator using PWM control generally employs slope compensation to avoid the occurrence of sub-harmonic oscillation.
To perform the slope compensation, a liner slope voltage is generally added to a converted slope voltage converted from a current value of an inductor to a voltage value. Further, a slope voltage having a high-order voltage wave with respect to time may be added to obtain a stable operation of an error amplifier.
FIG. 1 is a circuit diagram of a conventional switching regulator 100. The switching regulator 100 includes a slope voltage compensation circuit 20 and a switching device 21. The slope voltage compensation circuit 20 includes a current transformer 22, a diode 23, resistors 24 and 26, and a capacitor 25. The switching device 21 is formed of an NMOS transistor.
In the conventional switching regulator 100 of FIG. 1, a current flowing through the switching device 21 is detected by the current transformer 22. A current proportional to the current flowing through the switching device 21 is drawn from a secondary side of the current transformer 22 to charge the capacitor 25 through the diode 23 and the resistor 24.
FIG. 2 illustrates waveforms of the slope voltage compensation circuit 20 of the switching regulator 100 of FIG. 1.
When the switching device 21 is turned on based on a drive signal from a switching control circuit 27, the switching device 21 has a current increasing linearly on time as shown by a waveform (a) in FIG. 2. A proportional current proportional to the current of the switching device 21 is induced at a secondary side of the current transformer 22. Then, the induced current charges the capacitor 25 through the diode 23 and the resistor 24. A charging voltage of the capacitor 25 increases over time with a secondary curved slope as shown by a waveform (b) in FIG. 2. Therefore, the slope voltage Vslope is an output signal of the slope voltage compensation circuit 20 and is output from a connection node of a cathode of the diode 23 and the resistor 24, with a summation of voltage drop value at the resistor 24 and a charge-up voltage value of the capacitor 25.
As the slope voltage Vslope has a secondary curved slope as shown by a waveform (c) in FIG. 2, the switching regulator 100 has enough of a margin against the occurrence of sub-harmonic oscillation to achieve stable operation. However, in this configuration, it is difficult to obtain a slope voltage with a flexible slope angle over time with a combination of a slope angle of a linear part and a slope angle of a secondary curved part, because the slope voltage is based directly on the current of the switching device 21. In other words, it may not be possible to obtain a desired slope voltage. Further, the switching regulator 100 shown in FIG. 1 cannot be made compact and cannot be integrated onto an IC (integrated circuit) because the current transformer is necessary in this circuitry.
As described with reference of FIG. 1, the switching regulator 100 shown in FIG. 1 employs an integration circuit to integrate the current of the switching element 21. However, there are other switching regulators that utilize a saturation characteristic of a transistor to generate a non-linear slope voltage. Such a switching regulator that generates the non-linear slope voltage using the saturation characteristic of the transistor includes a constant current source, a MOS transistor and a capacitor connected to the MOS transistor. The non-linear slope voltage is generated by controlling a gate of the MOS transistor. However, a drawback of such switching regulator is that it requires a MOS transistor with a large size and a dedicated integrating circuit to generate the non-linear slope voltage.